EP2296844B1 - Aluminum alloy strips for brazed heat exchanger tubes - Google Patents

Abstract

The invention relates to a brazing metal sheet consisting of an aluminum alloy core sheet coated on at least one surface with a cladding layer forming a sacrificial anode, and is characterized in that said cladding layer consists of an aluminum alloy having a chemical composition, as expressed in weight percent, of: Si: > 2.0 – 7.0 Fe < 0.5 Cu < 1.0 Mn: 1.0 – 2.0 Mg < 0.5 Zn: 1.0 – 3.0 Cr < 0.25 Ni < 1.5 Ti < 0.25 Co < 1.5 V, In, Sn, Zr, Sc < 0.25 each; other elements < 0.05 each, and 0.15 total. The invention also relates to a heat exchanger tube produced by the bending and brazing of a brazing metal sheet, the sacrificial anode-forming cladding layer of which constitutes the inner coating of the tube, or the “inner liner”.

Description

Field of the invention

The invention relates to the field of aluminum alloy strips for the manufacture of heat exchangers, in particular those used for cooling engines, heating or air conditioning of the passenger compartment, or for any other heat exchange function. , in motor vehicles.

The latter, in particular those used for engine cooling radiator tubes or heating radiators, are generally coated on the outer face of a solder alloy and on the inner face of an alloy intended for protection against corrosion. corrosion and erosion by the coolant. The invention more particularly relates to these coated strips, which are part of what are called brazing sheets, used for the heat exchanger tubes, in contact with the cooling fluid.

State of the art

Aluminum alloys are now mainly used in the manufacture of heat exchangers for the automobile because of their low density, which allows a weight gain, especially compared to copper alloys, while ensuring good thermal conduction, ease of implementation and good resistance to corrosion.

All aluminum alloys referred to in the following are designated, unless otherwise indicated, in the terms defined by the "Aluminum Association" in the "Registration Record Series" which it publishes regularly.

The exchangers comprise tubes for the circulation of the internal heating or cooling fluid and fins or spacers to increase the heat transfer between the internal fluid and the external fluid, and their manufacture is done either by mechanical assembly or by soldering.

In the most common case of the solder joint, the core sheet constituting the tube, generally aluminum alloy AA3xxx series, is coated on the outer face, in contact with the fins, an alloy says brazing, usually from the AA4xxx series. This configuration is illustrated by the figure 1 , diagram 1a, the core plate bearing the mark 2 and the brazing alloy the mark 1.

It has the advantage of melting at a temperature below the melting temperature of the core and, by applying a thermal brazing cycle, to be able to create a connection between the two materials to braze, fins and outside of the tube.

The core sheet may also be coated on its inner face with a protective layer 3 against corrosion and erosion by the coolant or coolant. This configuration is illustrated by the figure 1 , diagram 1b.

This last layer, most often also in the form of colaminated sheet metal, is known in the art as the "inner-liner"; it is generally made of an alloy of the AA7xxx series.

The most widely used internal plating alloy to date is AA7072.

Indeed, because of its relatively high zinc content, on average 1.05% by weight percentage, its corrosion potential is lower than that of the core, typically Al-Mn-Cu type alloy, which allows it to ensure its role of sacrificial anode.

Examples of core Al-Mn-Cu alloys are alloys 3916 and 3915, respectively described in US Pat. EP 1075935 and the demand EP 1413427 of the applicant, and whose compositions specified below in percentages by weight, excluding impurity elements limited to 0.05% each and 0.15% in total: Yes Fe Cu mn mg Zn Ti 3916 0.15-0.30 <0.25 0.5-1.0 1.0-1.4 <0.01 <0.2 <0,1 3915 0.15-0.30 <0.25 0.5-1.0 1.0-1.4 0.10-0.35 <0.2 <0,1

However, because of its solidus temperature, of the same order as that of commonly used core alloys, about 640 ° C, AA7072 alloy does not participate in brazing.

Moreover, for the sake of efficiency of the heat exchange, more and more are substituted for the single rolled tubes welded, so-called section B tubes, as represented in FIG. figure 2 .

They are obtained by folding, in particular from a band or brazing sheet as defined above, that is to say with a solder plating on its external face, and a sacrificial veneer on its internal face.

However, the brazing of this type of tube is difficult, particularly at the outside of the foot 6 as represented inside the circle. figure 2 , left view.

Indeed, obtaining a correct solder joint in this zone requires the addition of a large quantity of brazing alloy 4 from plating 1, whereas the available external volume of said brazing alloy is limited and located at the sole level of the center of the foot as illustrated by the figure 2 , left view, in the center of the circle as well as in diagrams 1a and 1b.

One of the known solutions to this problem is to increase the thickness of outer plating 1 to result in a larger input in zone 6.

It will be recalled that, typically, in the case of a rolled-welded tube which is not subject to this problem, the external plating corresponds to 10% of the total thickness of the brazing sheet against 10% for the internal plating. , for a total thickness generally between 200 and 300 microns.

In the case of a B-section folded-brazed tube, in order to increase the thickness of the external brazing alloy plating 1, it is necessary either to increase the total thickness of the brazing sheet while maintaining the abovementioned percentages, or increase the percentage of constant thickness external plating of the solder plate.

These two solutions are clearly not satisfactory.

The first is totally contrary to the general trend in the automotive field, and more particularly in the field of heat exchangers, which consists in reducing the thickness of the components as much as possible.

The second involves the reduction of the thickness of the core which precisely ensures mainly the mechanical strength as well as the corrosion resistance of the material.

Another known solution is to use a solder alloy of AA4045 or AA4343 type for internal plating or "inner-liner". But such an alternative inevitably results in an unacceptable drop in corrosion resistance, in particular as measured by the so-called "OY" test known to those skilled in the art and described later.

Finally, the requests JP2005037062 Toyo Radiator, JP2004217982 and JP2004217983 from Sumitomo Light Metal describe alternative solutions consisting essentially of folds of the brazing sheet in the area of the center of the foot so as to make contact between two parts of the external veneer and thus guarantee soldering.

Such solutions have, in particular, just as the first described, the disadvantage of inducing a surplus of material used and an increase in the overall size of internal section of constant fluid circulation.

Problem

The object of the invention is to solve these soldering difficulties without surplus of material used nor of size or weight while ensuring resistance to internal corrosion at least equivalent to that obtained in the case of a brazing sheet of the art. former whose inner layer is made of an alloy of the AA7xxx series.

Object of the invention

• Si : > 2,0 - 7,0 Fe < 0,5 Cu < 1,0 Mn : 1,0 - 2,0 Mg < 0,5 Zn : 1,0 - 3,0 Cr < 0,25 Ni < 1,5 Ti < 0,25 Co < 1,5 V, In, Sn, Zr, Sc < 0,25 chacun, autres éléments < 0,05 chacun et 0,15 au total.

The subject of the invention is a brazing sheet consisting of an aluminum alloy core sheet coated on at least one face with a sacrificial anode-forming veneer layer, characterized in that said veneer layer consists of an aluminum alloy of chemical composition, expressed in percentages by weight:

• If:> 2.0 - 7.0 Fe <0.5 Cu <1.0 Mn: 1.0 - 2.0 Mg <0.5 Zn: 1.0 - 3.0 Cr <0.25 Ni < 1.5 Ti <0.25 Co <1.5 V, In, Sn, Zr, Sc <0.25 each, other elements <0.05 each and 0.15 in total.

According to an advantageous embodiment, the core sheet is made of aluminum alloy AA3xxx series and preferably alloy 3915 or 3916.

Similarly, the brazing sheet is generally coated on the outer face, opposite to the sacrificial anode plating layer, a AA4xxx series aluminum alloy brazing layer.

According to a preferred embodiment, the one or more layers, of plating, brazing and core plate, are assembled by bonding.

Finally, the subject of the invention is also a heat exchanger tube made by folding and brazing from a brazing sheet as described above, the sacrificial anode cladding layer of which constitutes the inner lining of the tube or " inner-liner ".

Description of figures

The figure 1 represents, according to FIG. 1a , a two-layer brazing sheet, the core sheet bearing the reference 2 and the brazing alloy (which may also be the cladding layer according to the invention), the reference 1, and according to FIG. 1b , a three-layer solder plate, the core sheet bearing the mark 2, the solder mark 1 and the sacrificial anode veneer layer 3.

• A gauche un tube en B obtenu par pliage d'une tôle de brasage constituée d'une tôle d'âme revêtue sur une face dite externe d'un alliage de brasage 1 et sur sa face interne d'un alliage de placage 2 ayant comme unique rôle de protéger l'âme contre la corrosion, mais ne participant pas au brasage.

The figure 2 schematically represents:

• On the left a tube B obtained by folding a solder plate consisting of a core sheet coated on a so-called outer face of a solder alloy 1 and on its inner face of a plating alloy 2 having as unique role of protecting the core against corrosion, but not participating in brazing.

Figure 1a shows an enlargement of the foot center area surrounded by the brazing alloy at 4, prior to the soldering operation.

Figure 1b shows the same area after soldering, with 5 in thick black and the solder joint and in 6 the area having soldering difficulties.
On the right, the same schematization of the tube B, but according to the invention, the inner face being coated with a layer of plating alloy 3 protecting the core against corrosion by sacrificial anode effect but participating in soldering.

In FIG. 2a , before brazing, the brazing alloy of the external plating, but also that of the internal plating, appears as before at 4, and in FIG. 2b , after brazing, the brazing joint 5 which also covers the zone previously difficult to weld. brazed.

The figure 3 illustrates the "V" test used to evaluate the solderability, with 3a a view from above and 3b a side view.

The plate 2 consists of the material to be tested. The "V" 1 consists of an alloy strip of the AA3xxx type, for example AA3003.

The figure 4 schematizes the classification mode from A to D of the quality of the solder joints made during the V-test, after cuts perpendicular to the "V" branch planes and the plane of the plate, then metallographic observation.

Description of the invention

• des performances de comportement en corrosion, notamment évaluées par le test dit « OY » connu de l'homme du métier et décrit au chapitre « Exemples », au moins identiques à celles d'une même tôle utilisant pour cette couche un alliage du type AA7072
• une température de fusion de la dite couche autorisant un brasage aisé.

The invention consists in the use, for the sacrificial anode plating layer of a brazing sheet, of an alloy whose composition is optimized in order to achieve:

• corrosion behavior performance, in particular evaluated by the so-called "OY" test known to those skilled in the art and described in the "Examples" chapter, at least identical to those of the same sheet using for this layer an alloy of the AA7072 type
• a melting temperature of said layer allowing easy soldering.

• Si : > 2,0 - 7,0 Fe < 0,5 Cu < 1,0 Mn : 1,0- 2,0 Mg < 0,5 Zn : 1,0 - 3,0 Cr < 0,25 Ni < 1,5 Ti < 0,25 Co < 1,5 V, In, Sn, Zr, Sc < 0,25 chacun, autres éléments < 0,05 chacun et 0,15 au total.

More precisely, the family in question is characterized by the following chemical composition, expressed in percentages by weight:

• If:> 2.0 - 7.0 Fe <0.5 Cu <1.0 Mn: 1.0 - 2.0 Mg <0.5 Zn: 1.0 - 3.0 Cr <0.25 Ni < 1.5 Ti <0.25 Co <1.5 V, In, Sn, Zr, Sc <0.25 each, other elements <0.05 each and 0.15 in total.

• Si constitue l'élément majeur quant à l'influence sur l'aptitude au brasage. Sa teneur doit être strictement supérieure à 2% car jusqu'à cette valeur, l'aptitude au brasage n'est pas satisfaisante; par contre, au delà de 7 %, la protection contre la corrosion par effet d'anode sacrificielle n'est plus suffisante. De préférence, sa teneur est comprise entre 3 et 5 % et encore plus préférentiellement entre 3 et 4 %.
• Fe constitue de façon générale une impureté pour l'aluminium ; les phases au fer constituent des sites privilégiés d'initiation de piqûres de corrosion. De ce fait, sa teneur doit être inférieure à 0,5 % et plus préférentiellement à 0,3 %.
• Cu a un effet favorable sur la résistance mécanique mais il augmente également le potentiel de corrosion réduisant ainsi l'effet d'anode sacrificielle. Par sa répartition non homogène au sein de l'alliage, il peut également accroître les risques de corrosion galvanique. De ce fait, sa teneur doit être limitée à 1 % et plus préférentiellement à 0,8 %. Dans certains cas, il peut même être souhaitable de limiter la teneur en cuivre à celle d'une impureté, soit 0,05 %, afin d'éviter des phénomènes de dissolution puis re-précipitation du cuivre à la surface.
• Mn est un élément durcissant ; il a un effet positif sur la résistance après brasage par durcissement en solution solide et sous forme de fins précipités. En dessous de 1%, le durcissement est insuffisant. Par contre, au-delà de 2 %, il donne lieu à la formation de phases intermétalliques grossières très défavorables à la coulabilité de l'alliage.
• Mg a une influence positive sur la résistance mécanique, mais il est néfaste à la brasabilité, dans la mesure où il migre à la surface du placage et, surtout dans le cas du brasage du type Nocolok^®, il vient former une couche d'oxyde qui modifie dans un sens défavorable les propriétés de la brasure. Pour cette raison, sa teneur doit être limitée à 0,5 % et mieux, dans le cas du brasage sous flux à 0,3 %. Pour certaines applications difficiles, il peut être nécessaire de réduire sa teneur à celle d'une impureté, soit 0,05 %, voire même, conformément au brevet EP 1075935 B1 de la demanderesse, à 0,01 %.
• Zn contribue de façon majeure à l'effet sacrificiel de la couche de placage. En dessous de 1 %, cet effet est insuffisant, et au-delà de 3 %, l'effet est trop marqué pour assurer une protection suffisamment durable. La fourchette préférentielle est de 1,5 à 2,5%.
• Ni et Co peuvent être ajoutés jusqu'à une teneur de 1,5 % pour améliorer les caractéristiques mécaniques de l'alliage ainsi que sa résistance à la corrosion, en particulier en milieu basique (pH supérieur à 9).
• Ti peut être ajouté jusqu'à une teneur de 0.25 % pour améliorer le comportement en corrosion de l'alliage. Le titane se concentre sur des couches parallèles à la direction de laminage, ce qui favorise une corrosion latérale plutôt qu'une corrosion par piqûres pénétrantes.
• Cr, V, Zr et Sc sont des éléments anti-recristallisants qui améliorent notamment la résistance mécanique après brasage, mais agissent également de manière favorable sur la résistance à la corrosion. Leur addition est possible jusqu'à une teneur de 0.25 % chacun.
• Enfin In et Sn ont un effet sacrificiel analogue à celui du Zn et peuvent être ajoutés jusqu'à une teneur de 0,25 % chacun.

The concentration ranges imposed on the constituent elements of each alloy can be explained by the following reasons:

• If constitutes the major element as to the influence on the aptitude to the soldering. Its content must be strictly greater than 2% because up to this value, the solderability is not satisfactory; on the other hand, beyond 7%, protection against sacrificial anode corrosion is no longer sufficient. Preferably, its content is between 3 and 5% and even more preferably between 3 and 4%.
• Fe is generally an impurity for aluminum; the iron phases are preferred sites for initiation of pitting corrosion. As a result, its content must be less than 0.5% and more preferably less than 0.3%.
• Cu has a favorable effect on the mechanical strength but it also increases the corrosion potential thus reducing the effect of sacrificial anode. By its uneven distribution within the alloy, it can also increase the risk of galvanic corrosion. As a result, its content must be limited to 1% and more preferably to 0.8%. In some cases, it may even be desirable to limit the copper content to that of an impurity, or 0.05%, in order to avoid phenomena of dissolution and re-precipitation of the copper on the surface.
• Mn is a hardening element; it has a positive effect on post-soldering resistance by hardening in solid solution and in the form of fine precipitates. Below 1%, curing is insufficient. On the other hand, beyond 2%, it gives rise to the formation of coarse intermetallic phases which are very unfavorable to the flowability of the alloy.
• Mg has a positive influence on mechanical strength, but it is detrimental to solderability, as it migrates to the surface of the veneer and, especially in the case of Nocolok ^® type soldering, it forms an oxide layer which modifies in an unfavorable direction the properties of the solder. For this reason, its content must be limited to 0.5% and better, in the case of brazing under a flow of 0.3%. For some difficult applications, it may be necessary to reduce its content to that of an impurity, or 0.05%, or even, according to the patent EP 1075935 B1 of the plaintiff at 0.01%.
• Zn contributes in a major way to the sacrificial effect of the plating layer. Below 1%, this effect is insufficient, and beyond 3%, the effect is too marked to ensure a sufficiently durable protection. The preferred range is 1.5 to 2.5%.
• Ni and Co can be added up to a content of 1.5% to improve the mechanical characteristics of the alloy as well as its resistance to corrosion, in particular in basic medium (pH greater than 9).
• Ti can be added up to a content of 0.25% to improve the corrosion behavior of the alloy. Titanium focuses on layers parallel to the rolling direction, which promotes lateral corrosion rather than pitting corrosion.
• Cr, V, Zr and Sc are anti-recrystallizing elements which notably improve the mechanical strength after brazing, but also act favorably on the corrosion resistance. Their addition is possible up to a content of 0.25% each.
• Finally In and Sn have a sacrificial effect similar to that of Zn and can be added up to a content of 0.25% each.

In its details, the invention will be better understood with the aid of the following examples, which are however not limiting in nature.

Examples

Several core alloy plates 3916 have been cast (according to the patent EP 1075935 above) and solder alloy AA4045 as well as seven sacrificial anode plating alloy plates and an AA7072 alloy plate used as a reference.

The composition of the six plating alloys 1 to 6 according to the invention as well as an alloy 7 with a Si content of 2.0%, excluding the invention, is shown in Table 1 below. Table 1 Alloy Yes Fe Cu mn Zn 1 2.5 0.15 - 1.15 1.4 2 3 0.15 - 1.15 1.4 3 3.5 0.15 - 1.15 1.4 4 2.5 0.15 0.4 1.15 1.6 5 3.5 0.15 0.4 1.15 1.6 6 2.5 0.15 0.65 1.35 1.8 7 2.0 0.15 - 1.15 1.4

Assemblies have been made from these plates such that each thickness of solder alloy on one face and plating alloy on the other side of the alloy core plate 3916 represents 10% of the total thickness.

Assemblies have also been made in the same way with, on each face, the brazing alloy AA4045.

These assemblies were hot-rolled and then cold-rolled to produce 0.25 mm thick plated strips. These strips were then subjected to a restoring treatment of 2 hours at 280 ° C. after temperature rise at a speed of 45 ° C./min.

The test tube described in figure 3 was used to evaluate the solderability of these materials.

The "V" consists of a bare strip of alloy 3003, in the state H24, and of thickness 0.3 mm. A degreasing treatment of 15 minutes at 250 ° C. is applied to the metal to be brazed. A Nocolok ^® flux is then deposited on the metal sheet 2 to be soldered. Brazing is done in a double-walled glass furnace that allows liquid solder movements and joint formation to be visualized during processing. The thermal cycle consists of a temperature rise up to 600 ° C with a speed of about 20 ° C / min, a 2 min hold at 600 ° C, and a descent to about 30 ° C / min. Everything is done under continuous nitrogen sweep, with a flow rate of 8 l / min.

The results are first qualified by a score of A to E assigned according to a visual inspection according to the following scale: Rating awarded AT B VS D E Length of joint formed with respect to the total length of the V 100% 90% 75% 50% 0%

The results obtained are shown in Table 2, the tests having been repeated four times for each plating alloy. Table 2 Alloy 4045 7072 1 2 3 4 5 6 7 Rating of seals AT E AT AT AT AT AT AT D

Metallographic examinations of the V-shaped solder joints were also performed after cuts perpendicular to the "V" branch planes and the plate plane, and the quality of the joints was classified according to the scale as illustrated. in figure 4 .

The results obtained are shown in Table 3 below: Table 3 Alloy 4045 7072 1 2 3 4 5 6 7 Rating of seals AT D VS B AT B AT VS D

These tests show that alloys 3 and 5, with a Si content of 3.5%, have the thickest solder joints, followed by alloys 2 and 4, for a content of 3 and 2.5%, and finally 1 and 6, for a content of 2.5%.

For all these cases, the solderability appears to be better than that of the AA7072 reference alloy. The alloy 7, with a Si content of 2.0%, does not provide a sufficiently significant improvement over the reference case.

Of course, in the case of 4045 alloy plating on both sides, the solderability is excellent.

The internal corrosion resistance, either on the sacrificial anode plating layer, was also evaluated by means of a so-called "OY" solution corrosion test.

The test carried out consists in maintaining, for 3 days at 88 ° C., a sample of the brazing sheet, typically of dimensions 60 × 100 mm, by protecting the non-examined face, in a liquid solution containing numerous ionic species (sulphates, chlorides, ferric, cupric).

The ionic concentrations of this solution are specified in Table 4 below: Table 4 Ion Ppm concentration Salts used Mass introduced in mg Cl ^- 195 NaCI 2272.7 SO4 ^2- 60 Na ₂ SO ₄ , 10H ₂ O 1961.7 Fe ³⁺ 30 FeCl ₃ , 6H ₂ O 1452 Cu ²⁺ 1 CuSO ₄ , 5H ₂ O 39.9

The corrosion resistance is quantified by pitting depth measurements by a differential focusing technique in optical microscopy and qualified by cross-sectional metallographic observations.

The results presented in Table 5 below correspond to the average of the 7 deepest stings. Table 5 Alloy 4045 7072 1 2 3 4 5 6 7 Average depth (μm) 122 86 85 87 92 81 85 77 86

The depths of bites recorded, for the six alloys according to the invention as well as the alloy 7, are of the same order as those measured in the case of the reference alloy AA7072.

The metallographic sections made on these samples after the "OY" test show that the corrosion mode of the sheets according to the invention is the same as that of the reference plates plated with alloy AA7072, that is to say lateralized, and that the maximum depth of pitting is comparable.

In the case of alloy plating 4045 on both sides, the average pitting depth is 122 μm, 50% greater than for reference 7072 and the alloys according to the invention.

The corrosion behavior of the sheets according to the invention is therefore at least equivalent to that of a brazing sheet of the prior art whose inner layer is made from an alloy of the AA7072 series, the most commonly used internal plating alloy.

On the other hand, in accordance with the intended purpose, these sheets are perfectly suitable for brazing also on the sacrificial anode plating layer according to the invention.

Claims (15)

1. A brazing sheet formed from an aluminium alloy web metal sheet coated on at least one surface with a plating layer forming a sacrificial anode, characterised in that said plating layer is formed from an aluminium alloy with a chemical com position expressed in percentage by weight:

   Si:>2.0-7.0 Fe<0.5 Cu<1.0 Mn:1.0-2.0 Mg<0.5 Zn:1.0-3.0 Cr<0.25 Ni<1.5 Ti<0.25 Co<1.5 V,In,Sn,Zr,Sc<0.25

   each, other elements <0.05 each and 0.15 in total.
2. The brazing sheet according to claim 1, characteris ed in that the Si content of the plating layer is included between 3.0 and 5.0 %.
3. The brazing sheet according to claim 2, characterised in that the Si content of the plating layer is included between 3.0 and 4.0 %.
4. The brazing sheet according to one of the claims 1 to 3, characterised in that the Fe content of the plating layer is lower than 0.3 %.
5. The brazing sheet according to one of the claims 1 to 4, characterised in that the Cu content of the plating layer is lower than 0.8 %.
6. The brazing sheet according to one of the claims 1 to 5, characterised in that the Cu content of the plating layer is lower than 0.05 %.
7. The brazing sheet according to one of the claims 1 to 6, characterised in that the Mg content of the plating layer is lower than 0.3 %.
8. The brazing sheet according to one of the claims 1 to 7, characterised in that the Mg content of the plating layer is lower than 0.05 %.
9. The brazing sheet according to one of the claims 1 to 8, characterised in that the Mg content of the plating layer is lower than 0.01 %.
10. The brazing sheet according to one of the claims 1 to 9, characterised in that the Zn content of the plating layer is included between 1.5 and 2.5 %.
11. The brazing sheet according to one of the claims 1 to 10, characterised in that the plating layer is applied to one surface of the series AA3xxx aluminium alloy web metal sheet.
12. The brazing sheet according to claim 11, characterised in that the plating layer is applied to a surface of the 3915 alloy web metal sheet.
13. The brazing sheet according to claim 11, characterised in that the plating layer is applied to a surface of the 3916 alloy web metal sheet.
14. The brazing sheet according to one of the claims 11 to 13, characterised in that the other surface of the web metal sheet is coated with a layer of series AA4xxx brazing aluminium alloy.
15. A heat exchanger tube produced by plating and brazing from a brazing sheet according to one of the claims 1 to 14, characterised in that the plating layer forming a sacrificial anode constitutes the internal coating of the tube or the inner liner.

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